MFC r322237: MFV r322236: 8126 ztest assertion failed in dbuf_dirty due to dn_nlevels changing

illumos/illumos-gate@dcb6872c565819ac88acbc2ece999ef241c8b982
https://github.com/illumos/illumos-gate/commit/dcb6872c565819ac88acbc2ece999ef241c8b982

https://www.illumos.org/issues/8126

The sync thread is concurrently modifying dn_phys->dn_nlevels
while dbuf_dirty() is trying to assert something about it, without
holding the necessary lock. We need to move this assertion further down
in the function, after we have acquired the dn_struct_rwlock.

Reviewed by: Pavel Zakharov <pavel.zakharov@delphix.com>
Reviewed by: Serapheim Dimitropoulos <serapheim@delphix.com>
Approved by: Robert Mustacchi <rm@joyent.com>
Author: Matthew Ahrens <mahrens@delphix.com>